Only the proteins that the bacteria escherichia coli (e. coli) needs are synthesized at any given time. for example, when lactose is available in the environment, e. coli will express the lac genes that allow it to break down lactose to obtain energy. when lactose is not present, these genes are not expressed.

this prevents the lac genes in the dna of e. coli from being expressed most of the time.
a. the lac repressor
b. the lac operator
c. the lac promoter
d. the lac locator